The present invention relates to a communication control device and a communication control method for use in transmitting ATM (Asynchronous Transfer Mode) cells in an ATM network.
In general, the ATM network of this kind includes a plurality of ATM terminals and an ATM server which are connected to each other via ATM switches. Normally, each of the ATM server and each of the ATM terminals is provided with a communication control device, as an ATM device, for transmitting ATM cells.
From the ATM terminals and the ATM server, ATM cells each having a header and a payload are sent to the ATM switches or the like via VCs (virtual channels) assigned to the ATM terminals and the ATM server. For controlling the data transmission rate of the ATM cell, a CBR (constant bit rate) mode is normally used. In the CBR mode, when the ATM cells are transmitted using a certain VC, a peak rate representing a minimum interval between the ATM cells in the same VC and a mean rate representing a mean value of transmission rates for a long time are fixedly determined before transmission of the ATM cells, and these rates are held unchanged during transmission of the ATM cells via the VC. As appreciated from this, the same VC is assigned in a time-division fashion at time intervals, that is, at a constant rate determined by the peak rate and the mean rate.
In the CBR mode, for keeping the foregoing rate and controlling data transmission timings among the ATM terminals and the ATM server in the network, a traffic shaper is provided at each of the ATM devices of the ATM terminals and the ATM server. This means that, during transmission of the ATM cells via the particular VC, the particular traffic shaper is fixedly connected. Thus, as many traffic shapers are required as.
Further, in the CBR mode, when the cells are transmitted at peak rates via a plurality of the VCs and full on the transmission line, it is possible that the cells can not be transmitted via another VC on the same line. Accordingly, when employing the CBR mode, it is necessary to assume in advance that a number of the VCs are used on the same line, and set a peak rate of each VC with a margin. Thus, the CBR mode h as a drawback where vacancies occur on the line or a drawback where a VC which can not transmit the cells is generated.
On the other hand, instead of the CBR mode an ABR (available bit rate) mode has been proposed as a de facto standard by an organization called The ATM Forum. In the ABR mode, since a peak rate of the VC c an be changed dynamically during transmission, cells can be newly transmitted via another VC by adjusting a band of the transmission line, that is, a transmission density. Accordingly, in the ABR mode, as compared with the CBR mode, since the peak rate can be freely changed, the transmission density can be flexible, and thus, the high-density transmission can be achieved with fewer virtual channels which can not transmit the cells.
However, in practice, specific operations and structures of the ATM device suited for the ABR mode have not been proposed up to now.
Now, it is assumed that the ATM devices having the traffic shapers employed in the CBR mode are applied to the ABR mode. In this case, since each traffic shaper regulates a fixed bit rate, it is necessary, for dealing with a plurality of the bit rates, to provide a plurality of the traffic shapers, each corresponding to one of the bit rates, relative to one VC. Further, the software is necessary for dynamically switching among the traffic shapers at high speed during transmission on the same VC.
However, the switching based on the software takes time and can not achieve real-time processing.
Further, if the foregoing structure for the CBR mode is applied to the ABR mode, it can not deal an increment the number of the VCs. Specifically, due to the recent advancement in ATM technique, it has been required that one ATM device controls as many as 1,000 VCs. Now, it is assumed that the foregoing CBR mode structure is applied to the ABR mode where three bit rates are switched relative to one VC. In this case, three traffic shapers are necessary relative to one VC so that as many as 3,000 traffic shapers are required for one ATM device. This makes the structure of the ATM device complicated. Further, on a practical basis, it is difficult to realize as many as 3,000 traffic shapers on a single LSI chip.
As appreciated from the foregoing, it is practically difficult to apply the software and the hardware of the CBR mode to the ABR mode with respect to both software and hardware.